1. Field of the Invention
The present invention relates, in general, to an apparatus for forming longitudinal thermal-fatigue cracks on a tubular test piece in order to verify a technique of non-destructive inspection, and more particularly, to an apparatus for forming longitudinal thermal-fatigue cracks, in which, unlike a prior art in which circumferential cracks are formed by maximizing z-axial (longitudinal) stress selected from x-, y- and z-axial stresses, the longitudinal thermal-fatigue cracks are generated by maximizing y-axial (circumferential) stress selected from x-, y- and z-axial stresses generated by mounting on a tubular test piece a cooling block adapted to allow cooling water to flow in an inner space thereof in order to control a direction (longitudinal or circumferential direction) in which cracks occur, thereby ensuring effectiveness in terms of verification of a technique of non-destructive inspection.
2. Description of the Related Art
As well known in the art, safety-grade equipment of a nuclear power plant that is in operation is inspected according to an inspection period using non-destructive inspections which take into consideration the material, shape, predicted defects, etc. of the equipment in order to secure soundness of the equipment and stability of the plant.
Since such inspection of the nuclear power plant that is in operation is restrictive on accessibility, and provides important results relevant to lifespan prediction and safety evaluation of the equipment, a case is made to apply the non-destructive inspection which has actually proven defect detectability and is of high reliability.
In particular, the defect detectability of ultrasonic testing (UT) and eddy current testing (ECT) has been demonstrated using simulated test pieces based on machining methods in use up to the present. However, many cases that cause misgivings about the accuracy of defect detectability have occurred.
The non-destructive demonstration test pieces requested by a technology level applied to the nuclear power plant (ASME Section XI, Appendix VII) must include geometrical defects, implants, weld solidification cracks, lack of fusion, mechanical fatigue cracks, electrical discharging machine (EDM) notches and holes, thermal fatigue cracks, intergranular corrosion cracks (IGCC), and so on.
Among them, the thermal fatigue cracks are formed on the test piece using the following four methods.
First, the test piece is mounted on an autoclave, and is repetitively heated to high or low temperature under proper tensile or compressive stress.
Second, the tensile and compressive stresses are repetitively applied to the test piece at constant temperature.
Third, the temperature is repetitively varied without applying a mechanical load to the test piece.
Fourth, the test piece is constrained at opposite ends thereof, and is maintained at a constant temperature which is obtained using a conductive member or a high-frequency induction heater near the conductive member. Then, the test piece is subjected to thermal stratification by contacting it with cooling water.
However, the thermal fatigue cracks of the test pieces made using the first, second and third methods of forming the thermal fatigue cracks on the test piece differ from those generated from the equipment of the nuclear power plant or the process industry that is being actually operated in many aspects, so that the effectiveness of the verification of the technique of the non-destructive inspection cannot be positively ensured.
Further, in the fourth case, the test piece can be made so as to form the cracks similar to those generated from the equipment of the actual nuclear power plant, but it cannot positively guarantee the control of directions where the cracks are formed.
In order to overcome this problem, the applicant is granted an improved apparatus for forming thermal fatigue cracks as disclosed in Korean Patent No. 10-0801 404.
The apparatus for forming thermal fatigue cracks, which the applicant has previously filed for and been granted, is composed of a heating unit having a conductive member attached to an outer circumference of one side of a tubular test piece and an induction heating coil disposed adjacent to the conductive member, a cooling unit having a cooling water pump forcibly injecting cooling water from a cooling water reservoir to an inner circumference of the tubular test piece using a cooling water hose, and a control unit controlling the heating unit and the cooling unit. With this configuration, the reliability of the verification of the technique of the non-destructive inspection can be increased by forming the thermal fatigue cracks having characteristics similar to the thermal fatigue cracks generated from the equipment of the nuclear power plant or the process industry that is being actually operated.
However, the apparatus for forming the thermal fatigue cracks granted to the applicant can merely verify the technique of the non-destructive inspection in terms of the thermal fatigue cracks formed in a circumferential direction, so that an environment similar to that where natural cracks occur cannot be created, and thus the resulting effectiveness cannot help but be restrictively guaranteed.
In detail, the conventional apparatus for forming the thermal fatigue cracks cannot arbitrarily determine the direction where the cracks are formed, and thus non-destructive inspectors cannot acquire a technique capable of discriminating signals depending on the direction of the cracks in terms of the direction (longitudinal and circumferential directions) of the natural cracks generated from the equipment of the nuclear power plant or the process industry that is being actually operated.